diff --git a/Modules/_struct.c b/Modules/_struct.c
index 796d168..2635af9 100644
--- a/Modules/_struct.c
+++ b/Modules/_struct.c
@@ -539,7 +539,7 @@
                         "ubyte format requires 0 <= number <= 255");
         return -1;
     }
-    *p = (char)x;
+    *(unsigned char *)p = (unsigned char)x;
     return 0;
 }
 
@@ -868,6 +868,7 @@
 {
     long x;
     Py_ssize_t i;
+    unsigned char *q = (unsigned char *)p;
     if (get_long(v, &x) < 0)
         return -1;
     i = f->size;
@@ -880,7 +881,7 @@
 #endif
     }
     do {
-        p[--i] = (char)x;
+        q[--i] = (unsigned char)(x & 0xffL);
         x >>= 8;
     } while (i > 0);
     return 0;
@@ -891,6 +892,7 @@
 {
     unsigned long x;
     Py_ssize_t i;
+    unsigned char *q = (unsigned char *)p;
     if (get_ulong(v, &x) < 0)
         return -1;
     i = f->size;
@@ -901,7 +903,7 @@
             RANGE_ERROR(x, f, 1, maxint - 1);
     }
     do {
-        p[--i] = (char)x;
+        q[--i] = (unsigned char)(x & 0xffUL);
         x >>= 8;
     } while (i > 0);
     return 0;
@@ -1087,6 +1089,7 @@
 {
     long x;
     Py_ssize_t i;
+    unsigned char *q = (unsigned char *)p;
     if (get_long(v, &x) < 0)
         return -1;
     i = f->size;
@@ -1099,7 +1102,7 @@
 #endif
     }
     do {
-        *p++ = (char)x;
+        *q++ = (unsigned char)(x & 0xffL);
         x >>= 8;
     } while (--i > 0);
     return 0;
@@ -1110,6 +1113,7 @@
 {
     unsigned long x;
     Py_ssize_t i;
+    unsigned char *q = (unsigned char *)p;
     if (get_ulong(v, &x) < 0)
         return -1;
     i = f->size;
@@ -1120,7 +1124,7 @@
             RANGE_ERROR(x, f, 1, maxint - 1);
     }
     do {
-        *p++ = (char)x;
+        *q++ = (unsigned char)(x & 0xffUL);
         x >>= 8;
     } while (--i > 0);
     return 0;